For various reasons (maintenance, pollution, fire risk, etc.), it is desirable to reduce the fraction of flight controls that are hydraulic in favor of controls that are electrical.
Unfortunately, the technological solution which consists in using electromagnetic motors in association with stepdown gear box forming means gives rise to equipment of mass that is too high.
To mitigate the drawbacks of electromagnetic motors, proposals have already made to use motors based on piezoelectric or electrostrictive materials suitable for presenting high energy densities and capable of withstanding high stresses, for example vibration motors or amplifier actuators.
In this respect, reference can advantageously be made to:
"Actionneurs--Des materiaux piezo-electriques pour les commandes du futur" [Actuators--piezoelectric materials for controls of the future], Usine nouvelle, Oct. 31, 1996, No. 2568;
"Des commandes de vole piezo-electriques" [Piezoelectric flight controls], Air et Cosmos/Aviation International, No. 1602, Feb. 28, 1997; and
"A new amplifier piezoelectric actuator for precise positioning and semi-passive damping", R. Le Letty, F. Claeyssen, G. Thomin--2nd Space Microdynamics and Accurate Control Symposium, May 13-16, 1997, Toulouse.
Nevertheless, the solutions shown in those publications are not satisfactory.
In particular, vibration motors cannot be used for primary controls since continuous operation would lead to the interface wearing too fast and to the most recent positions being maintained in the event of a power failure.
As for amplifier actuators, they require massive converter structures, thereby greatly reducing the initial advantage of lightness and energy density.
An object of the invention is to propose a piezoelectric or electrostrictive actuator structure which is rigid, lightweight, and capable of converting small unit piezoelectric or electrostrictive displacements into large displacements.
Although not to be taken into consideration when assessing the inventive step of the invention described below, since not yet published, it is appropriate to mention French patent application No. 97/12744 of Oct. 13, 1997 in the name of the Applicant company for a better understanding of the present invention.
That application describes an actuator which comprises a plurality of stacks of unit blocks of active piezoelectric or electrostrictive material, which blocks are juxtaposed in such a manner as to form a tubular structure constituted by superposed layers of blocks of active material having different polarization from one layer to another.
Separation strips which are distributed over the entire periphery of the structure and each of which extends over the full height thereof, separate successive unit blocks in the layers of the structure in pairs. Each separation strip is itself constituted by a plurality of separation elements superposed in the height direction of the structure. Each of those separation elements extends over two layers of active blocks and they are suitable for sliding over one another. They are of stiffness greater than that of the unit blocks of active material. By way of example, those separation strips are constituted by metal strips having slots distributed over their height, said slots separating successive separation elements.
As an illustration, a structure of that type is shown in developed form in FIG. 1. In this figure, the unit blocks of active material are referenced 1, and the separation elements are referenced 2. The arrows in the blocks 1 indicate their directions of polarization.
With such a structure, the actuator is subjected to twisting deformation when a voltage of alternating sign is applied to the metal separation strips.
This is shown in FIG. 2.
This twisting deformation can amount to 15.degree., or even more.
Making such an actuator requires the ceramics constituting the active blocks to be prestressed.
In the solution proposed by the above-mentioned patent application, the prestress is provided by rings of shape memory alloy that are electrically insulated from one another so as to avoid any short circuiting. Assembly then takes place in the manner shown in FIGS. 3a to 3c:
alternating prestress rings BP and insulating washers R are stacked at low temperature in an outer tube T (FIG. 3a); PA1 split metal separation strips are stuck along the generator lines of an inner cylindrical core N; PA1 the core is inserted with its strips into the tube formed by the prestress washers; PA1 ceramic blocks 1 are inserted into the housings thus defined between the rings, the strips, and the core; and PA1 the assembly is heated so as to cause the prestress rings BP to change phase, so that said rings then exert force on the metal strips. PA1 modules each comprising two active blocks on either side of a separation slab are installed between the separation slabs which are integral with the prestress structure of a stage; PA1 a prestress force directed radially towards the inside of the stage is applied to those separation slabs of said stage that are not integral with the prestress structure; PA1 the prestress structure of the following stage is positioned on the stage obtained in this way by placing the separation slabs which are integral with said prestress structure on the separation slabs of the preceding stage which are portions of the modules inserted between the separation slabs that are integral with the prestress structure of said stage; PA1 the separation slabs superposed in this way are bonded together; PA1 the prestress force exerted on the slabs of the preceding stage is removed; and PA1 assembly of the new stage is continued by installing modules, each comprising two active blocks on either side of a separation slab between the separation slabs that are integral with the prestress structure of the new stage. PA1 modules each comprising two active blocks on either side of a separation slab are installed between the separation slabs which are integral with the prestress structure of a stage; PA1 two stages obtained in this way are superposed while applying a prestress force which is directed radially towards the insides of said stages firstly on those separation slabs of one of the stages which are not integral with the prestress structure, and secondly on the separation slabs of the other stage which are superposed with said prestress separation slabs and which are integral with the prestress structure of said other stage; and PA1 the prestress separation slabs as superposed in this way are bonded together.
Nevertheless, such an implementation gives rise to several technical problems.
It is difficult to make compatible with conventional manufacturing tolerances that are conventional for ceramics. In particular, making ceramics with the tolerance that is required to ensure that the metal strips and the ceramic blocks are properly interfaced so as to have reliable and controllable prestresses available turns out to be difficult and gives rise to considerable extra cost.
In addition, the prestress rings give rise to extra weight which is penalizing for the actuator tube.
Also, working with materials that have shape memory gives rise to special problems associated with the hyperelastic behavior of such material and to the way in which their hyperelastic region moves as a function of temperature.